The present invention relates to swept lasers, and more particularly to real-time wavelength calibration for swept lasers with picometer accuracy.
Photonic networks have seen a virtual explosion in complexity as more and more enabling components become commercially available. Many of these components are active, such as distributed feedback (DFB) lasers and erbium-doped fiber amplifiers (EDFAs). Other components are passive, such as multiplexers/demultiplexers and fiber Bragg gratings (FBGs). Often the characteristic of greatest interest in these passive components is their spectral transmission and/or reflectivity.
To measure the spectral characteristics of passive optical components, the industry has settled on two different techniques. One uses a broadband (spectrally bright) source to illuminate the component at the input and measures the spectral content of the light that is either transmitted or reflected by using an optical spectrum analyzer (OSA). The other technique uses a tunable laser as input to the passive component and a broadband detector, such as a power meter, on the output. As the laser""s wavelength changes as measured by a wavelength meter, the power meter records differences in intensity and thus measures the wavelength-dependent transmission or reflectivity of the component.
Of these two techniques the tunable laser offers the best spectral resolution and dynamic range. Because of this it is becoming widely believed that the tunable laser method is the one most likely to succeed, though problems still remain. One of the most important problems is to achieve rapid, yet accurate, wavelength calibration. The most common configuration for this test bundles the tunable laser with a standard wavelength meter that is based on a Michelson interferometer. In this scenario the laser increments its wavelength and stops. The power meter reads the optical power and the wavelength meter measures the wavelength, and the process repeats.
The primary issue for this scenario is the time required to measure the wavelength with the wavelength meter. A typical Michelson interferometer needs many thousands of fringes to make an accurate wavelength measurement. Scanning this many fringes might take more than 50 milliseconds to acquire. Then the wavelength meter must take the fast Fourier transform (FFT) of the fringes and calculate the wavelengthxe2x80x94a process that might take another 50 milliseconds, for example. In this illustration it takes about 0.1 second to measure the wavelength of the tunable laser.
If the spectral characteristics of a passive component are tested over a range of 2 nanometers (2,000 picometers) and the wavelength is indexed in 2 picometer steps, the laser is stepped 1000 times and each step requires 0.1 second to perform the wavelength calibration. The total test time is about 100 seconds or 1.67 minutes. Scanning with 1 picometer resolution doubles the time, and if the scan is extended over a range of 20 nanometers the time increases an additional ten-fold. A 100 nanometer range scan would require 2.78 hours! To test hundreds or thousands of such passive components results in the test station becoming a bottleneck that limits production rates. After calibrating the laser at the beginning of a use period, the laser is swept without the wavelength meter for a while before recalibrating. The results are not as accurate as calibrating before each sweep, but it is a compromise between the time required for calibration and the desired accuracy of the results.
Therefore what is needed is a way to quickly measure the wavelength of a swept laser while doing it with great accuracy.
Accordingly the present invention provides a near real-time wavelength calibration for swept lasers by generating from the swept optical output an electrical signal that is cyclical with wavelength and by calibrating the electrical signal using known absorption lines within a defined range of wavelengths. One way to calibrate the swept lasers is to input the swept optical output into a polarizer that is coupled to one end and oriented at forty-five degrees to the eigen modes of a highly birefringent section of fiber. The other end of the highly birefringent fiber is coupled to a polarizing beam splitter which separates the orthogonally polarized modes. The outputs from the beam splitter are detected and the electrical outputs are composited to form the cyclical electrical signal. Another way to generate the cyclical electrical signal is to input the swept optical output into an unbalanced interferometer having a pair of unequal path lengths. The outputs from the two paths are input via a coupler to an optical receiver to obtain the cyclical electrical signal. For either way any point on the cyclical electrical signal corresponds accurately to the wavelength of the swept optical output at that point. In a parallel path a gas absorption cell containing a gas with known spectral absorption lines within the defined range of wavelengths receives the swept optical output and the spectral lines are detected by a detector to produce calibration references for the cyclical electrical signal at known wavelengths. Other points on the cyclical electrical signal are calibrated by interpolation between the known calibration references based on the phase difference between known spectral lines and the phase difference between the first known spectral line to the point desired.
The objects, advantages and other novel features of the present invention are apparent from the following detailed description when read in conjunction with the appended claims and attached drawing.